Electronic curve follower



Jan. 2, 1962 c, w. JOHNSON ELECTRONIC CURVE FoLLowER 5 Sheets-Sheet 1 Filed Oct. 26, 1956 Jn. 2, 1962 c. w. JoHNsoN ELECTRONIC CURVE FOLLOWER Filed Oct. 26., 1956 5 SheetsShee't 2 FIG.2.

To Y oEFLEcTloN AMPLlFlER FIGA.

TO X DEFLECTION AMPLIFIER INVENTORI CHARLES W. JOHNSON [BYWM 5 Sheets-Sheet 5 Filed Oct. 26, 1956 TIME INVENTOR CHARLES W. JOHNSON ATTORNE Jan. 2, 1962 Filed Oct. 26, 1956 C. W. JOHNSON ELECTRONIC CURVE FOLLOWER 5 Sheets-Sheet 4 INVENTOR'. CHARLES W. JOHNSON,

BYQA' ATTORNE Jan. 2, 1962 c. w. JOHNSON ELECTRONIC CURVE FOLLOWER 5 Sheets-Sheet 5 Filed Oct. 26, 1956 DISTANCE d ...mm-m

DISTANCE d O ZOSES.. QZOOmm Tial FIRST HAR MONIC FIRST AND SECOND HARMONIC INVENTOR' CHARLES W.JOHNSON Q2u H AT TORNE United States Patent 0 3,015,730 ELECTRNIC CURVE FOLLOWER Charles W. Johnson, Syracuse, NSY., assignor to General Electric Company, a corporation of New York Filed Oct. 26, 1956, Ser. No. 618,553 13 Claims. (Cl. Z50-202) This invention relates to an electronic curve follower which can also be used as a simulator or analog computer. More particularly, the invention relates to an electronic simulator or analog computer which utilizes a phase synchronized and frequency modulated oscillator as one of the elements thereof, which can resolve, synthesize and operate upon vector quantities and which is adapted to be used, for example, as a curve follower, as an element of a form recognition system, asa means of measuring various properties of curves, as a device for performing certain curve transformations and for preparing computer or control programs, or as an arbitrary function generator or the like.

Electromechanical curve following devices have in the past been used for such purposes as the automatic control of various machine tools. Typically, such devices comprise a photoelectric curve reading head mounted upon a mechanical support, the motion of which is controlled by error signals developed by the photoelectric reader as it traverses a curve. The motion of the head may then be used to derive information to control any desired machine tool. While these devices are well suited to their intended purpose, the electro-mechanical nature of the system imposes a very distinct limit on the speed at which a given curve may be read, which, in other applications, is often undesirable.

An electronic curve follower, eommonlyknown as the photoformer, has been used for some time in the analog computing arts as an arbitrary function generator. This system comprises an opaque mask which is placed over the lower portion of the face of a cathode ray tube. The upper edge of the mask represents, in an orthogonal x-y coordina-te system corresponding to the horizontal and vertical deflection axes of the tube, a plot of the function y;f(x) which one desires to generate. A linear sawtooth deflection voltage is` applied to the horizontal plates of the tube to generate the independent variable x and a bias voltage is applied to the vertical plates to initially position the spot of light formed by the electron beam at the top of the face of the cathode ray tube. A photocell is positioned to pick up light emitted from the face of the tube and to develop a voltage proportional to the intensity of this light. This voltage is applied to the vertical deilection plates in opposition to the bias voltage, so that the system is maintained in equilibrium when the spot of light rides along the upper edge of the opaque mask. The net voltage on the vertical deflection plate of the tube as the beam is swept horizontally by the linear x deflection voltage is then yan analog representation of the function y=f(x), the functional relationship being as determined by the contour of the upper edge of the mask.

While the photoformer is capable of operating speeds far greater than those of electromechanical curve followers, it obviously is not capable of following completely around all closed curves yor even following an open ended curve which may be multiple-valued. This, ofcourse, follows from the fact that the linear sawtooth used as the horizontal or x deflection sweep permits the system to trace out only open ended, single valued curves.

There are many applications wherein it would be desirable to have a curve following system which has both the high operating speeds of an electronic device and the flexibility as to the nature of the curve which may be read that is presently found only in electromechanical systems. These applications include, for example, form recognition or document reading systems, curve measuring systems, arbitrary function generators, and numerous applications in the control and computing arts, such, for example, as program preparation and curve transformation. One such desired curve following system is disclosed and claimed in the copending application S.N. 618,504, of Joseph W. Brouillette, Jr., and Charles W. Johnson, led concurrently herewith, now Patent No. 2,980,332, and assigned to the same assignee as the present application. In this system, which will hereinafter be referred to as the system of application S.N. 618,504, the desired speed and flexibility is achieved by the use of an electronic analog computer or simulator which can treat vector quantities in either D.C. component or A.C. polar form as representations, for example, of various geometrical properties of curves or dynamic properties of physical systems. The system utilizes some of the circuits and techniques which are prently used in general purpose electronic analog computers of the type described, for example, in the book Electronic Analog Computers, by G. A. Korn and T. M. Korn, published by McGraw Hill, New York, New York, 1952. The system further utilizes a small search circle intersecting with the curve to be traced in order to derive information from which an error signal may be obtained to servo-control the center of the search circle to follow a path determined by the curve. In the Brouillette-lohnson system the center of the search circle is caused to follow a path at a small predetermined distance away from the curve being read. While there are many applications in which this is a desirable feature, there are also applications in which it would be desirable to have such a curve following system wherein the search circle would center and ride directly on the curve being read.

It is therefore an object of this invention to provide a novel electronic curve follower utilizing a search circle which is directly centered upon a curve to be read and which is capable of following either closed or open ended curves which may be either single or multiple valued.

It is a further object of this invention to provide such a curve follower which will generate voltages representing various geometrical and mathematical properties of the curve being read.

It is a further object of this invention to provide a novel electronic computer or simulator utilizing a phase synchronized oscillator and means to sample the instantaneous value of the output of the oscillator in accordance with timing signals determined by the systems master oscillator.

It is a still further object of this invention to provide electronic apparatus and circuitry for simulating the motion of a particle.

Briey stated, in accordance with one exemplary embodiment of the invention, light from the screen of a cathode ray tube of a ying spot scanner is focused on a curve display means, which may be a transparent member having opaquely drawn thereon the curve to be investigated. The light transmited by the member is refocused on a photoelectric cell or transducer. The spot is caused to execute a search circle the diameter of which is small compared to the dimensions of the curve in question by a pair of deection voltages generated by a master oscillator at a constant carrier frequency and 90 phase difference. The intersection of the search circle with the curve produces two pulses per cycle of the carrier. When these pulses are ltered, their second harmonic is a maximum in amplitude when the search circle is centered on the curve and has a phase angle equal to the direction angle of the curve. This second harmonic may therefore be used to phase synchronize an oscillator the output of which represents the velocity to be imparted to the center of the search circle so that it will follow around the curve. When this oscillator output is processed and applied to the deection plates of the cathode ray tube there is formed a closed-loop, servo-system or simulator wherein various voltages are available representing, for example, certain geometrical properties of the curve.

While the novel and distinctive feature of the invention are particularly pointed out in the appended claims, a more expository treatment of the invention in principle and detail together with additional objects and advantages thereof, is afforded in the following description and accompanying drawings of a representative embodiment wherein like reference characters are used to indicate like parts throughout and in Which- FIGURE l is a block diagram of the system of the present invention.

FIGURES 2, through 6, 7a, 7b, 8, 9a, 9b, 9c, 10a, 10b, lla, 1lb and 13 are diagrammatic illustrations of various geometrical and electrical properties involved in the operation of the system of FIGURE l.

FIGURE l2 is a schematic circuit diagram of the phase synchronized oscillator shown in block form in FIG- URE 1.

Turning now to the drawings, FIGURE 1 is a block diagram of the system of the present invention, including an electron beam device shown as a conventional cathode ray tube 10. Tube l is equipped with any convenient deflection system which imparts vertical and horizontal components of motion to the electron beam in the tube in accordance With voltages applied to the deflection system. The deflection of the electron beam, of course, controls the position of the spot of light seen on the face of the tube when the electron beam strikes the phosphor screen thereof. The deliection system may, for example, be of the electrostatic type having horizontal and vertical deflection plates, as shown diagrammatically in FIGURE 2. It is convenient, for the purposes of this specincation, to consider the horizontal deflection voltage as representing the value of the x coordinate, and the vertical deection voltage as representing the value of the y coordinate of the spot S in a righthanded orthogonal cartesian coordinate system having its center or origin at the center C of cathode ray tube l0, and having its axes oriented along the two deflection axes. The position on the screen of the spot of light S at any instant may then be represented by a position vector P having an x component, px, and a y component, py. Such a representation logically assumes that the deflection system of the tube 10 is linear in the relation between applied voltage and amount of deflection. In fact this linearity is not necessary in all applications of the device, but the explanation of the system is claried by making this assumption for the present.

As is Well known in the art, any vector quantity may be specified by stating the value or magnitude of its two orthogonal components, or alternatively, by stating the direction angle and the scalar value, that is, the magnitude or length of the vector itself. By the direction angle of the vector is meant the angle between the vector and a reference vector Which is, by convention, taken to lie along the x axis. For the purposes of this specification, a directional vector quantity will be indicated by a capital or upper case letter underlined, whereas the scalar value or magnitude of the vector quantity will be indicated by the same capital letter without underlining. Orthogonal components will be indicated by corresponding small or lower case letters with appropriate subscripts. In other words, the position of the spot S shown in FIGURE 2 at the end of position vector P may be uniquely defined either by stating the values ofpX and py, the orthogonal components along the x and y axes respectively, or by stating the length or scalar value, P and the value of the direction angle A between P and the x axis. The latter form is generally called a polar representation of the vector, whereas the former is known as a component representation of the vector. Either one of the two equivalent ways of defining the same vector quantity may be more convenient than the other for a particular purpose. The process of transformation from the polar to the orthogonal component form of vector representation is commonly known as resolving the vector into its orthogonal components. The converse process of transforming from the component to the polar form may be termed synthesizing the vector. The latter term may also include any means of obtaining a desired vector in polar form.

If spot S moves in a straight line to S in one unit of time, as shown in FIG. 3, the new position may be similarly specified by the vector P. Additionally, the velocity of motion of the spot fr om S to S may be specified by a vector V. Vector P', of course, is the vector drawn from the-origin C f o the new position S'. Velocity vector V, on the other hand which in general equals AP/At, is here the vector drawn from S to S since At was specified to be one unit of time. The magnitude of length of V represents the average linear velocity or speed of tl@ spot which, as is well known, is equal to the distance traveled divided by the time. The velocity vector V however also has a direction as well as a magnitude. riis direction may be stated by specifying the angle between the vector V and the x axis or equivalently the angle qb between vector V and a line parallel to the x axis.

Thus, as with the position vector, the Velocity vector may be completely specified by stating its magnitude and its direction angle. Similarly, it like the position Vector, may alternatively be specified by stating the value of its x and y components, vX and vy, which are the projection of V on the x and y axes respectively, as shown in FIGURE 3. As is well known, the magnitude of these x and y component Values may be found from the vector V from the relations vX=V cos qb (1B) vy=V sin It is apparent that if the spot S moves in a straight line at constant speed, the vector V will remain constant in both magnitude and direction-If the speed changes along the same straight line, the magnitude of V will change but its direction angle will remain constant. If the magnitude of V remains unchanged while the spot moves in a curve ra-ther than in a straight line, then only the direction angle 1 changes and the spot may be said to be traveling along the curve at constant speed V. Of course, both the magnitude and direction of V may, in general, change simultaneously.

lf one repeatedly chooses the unit of time to be smaller and smaller, one second, one millisecond, one microsecond, etc., one will approach, in the limit, the instantaneous values of these vectors. This in effect is what is done by the well known methods of differential and integral calculus in terms of which the foregoing relationships may be stated as follows:

y=d/dt( P (2) E=fydt l 3) That is to say, velocity equals the derivative or rate of change of position with respect to time and conversely position equals the integral of velocity with respect to time. The computation of either the derivative or the integral of a vector quantity is an example of what is commonly termed performing an operation on the vector.

The voltages which are actually applied to the horizontal and vertical deection plates of tube 10 from detlection amplifiers 25 and 26 have values which represent, or in other Words, which are proportional to the components, pX and py of position vector P. This is to say, one volt, for example, may cause a d eection of the spot S of one centimeter (or some other unit of distance) on the face of the tube along the axis perpendicular to the deilection plates to which the voltage is applied. The factor of proportionality is commonly known as a scale factor. As shown in FIGURE 2, a positive voltage from the x deflection amplifier will move the spot a distance px to the right along the x axis and a positive voltage from the y deflection amplier will move the spot a distance py upwardly along the y axis. If both components are applied simultaneously, the net result is to move the spot S to the end of position vector P. Of course negative voltages would move the spot ino-pposite directions. Since the amount and direction of the deflection are proportional respectively to the magnitude and polarity of the applied deflection voltages, these voltages are herein called position voltages, pX and py respectively, as shown in FIGURES 1 and 2. ln a similar fashion, voltages which, when applied as inputs to electronic integrators, produce output voltages that are these above defined position voltages, will be called velocity voltages in accordance with Equation 3 above.

When the system is first energized by applying conventional power supplies (not shown), the voltage px and py are derived, via the deection amplifiers, from horizontal and vertical position potentiometers a and 2Gb, and from a search circle generator 21. It should be understood that potentiometers 2011 and 20h may be replaced by any convenient deection system or sweep generator which will cause the spot to initially intersect the curve. One particular form of initial Search sweep generator is, for example, disclosed and claimed in the above noted copending application S.N. 618,504. Furthermore, in practice slight disturbances in the System or charges accumulated in the condensers of integrators 54 and 55 (hereafter more fully treated) may be suiicient to cause the spot to initially drift into the curve. It is, however, convenient to use potentiometers 20a and Zibb to initially control the motion of the center of the search circle before it intersects the curve.

Search circle generator 21 may conveniently comprise a master oscillator 22a, which is preferably a crystal controlled oscillator, but may be any convenient means for lgenerating a stable alternating output of the form E sin wt, which is applied through potentiometer 23 to the y deflection amplifier 26. Here, E is the magnitude, that is, the peak or maximum value of the voltage, w is the angular frequency of the voltage, and t is time. Also, w equals 21rf, where 2nradians equals 360 and f is the frequency of alternation of the voltage in cycles per second. As s hown diagrammatically on an enlarge scale in FIG. 4, such an alternating or A.C. voltage, E sin wt, represents the vertical or y component of a Voltage vector E rotating counterclockwise at a frequency, f, equal to v'fz/ 21r. At time, t, equal to 0, the vector E will lie along the x axis and, in general, at any time, tfit will be at an angle, wt, to the x axis. When time, t, equals 21r/w, vector E will have made one complete revolution corresponding to one cycle of the A.C. voltage. Like any other vector, E has orthogonal x and y components which must be alternating in value in order to cause its rotation and which are, respectively, E cos wt and E sin wt.

The output, E sin wt, of master oscillator 22a is applied, as noted above, to the y deflection amplifier 26. In order to obtain the x components of the vector E, this output is also applied to an element 22h which Ina-y be any conventional network that produces a phase lead of 90 or 1r/2 radians of its output voltage compared to its input voltage. Element 22h, therefore, has an output voltage, E sin (wt-|-1r/2), which, as is well known, is equal to E cos wt. This output is the required x component of Vector E, and is applied through potentiometer 24 to the x deflect-ion amplifier 25 of tube 10. The combined effects of the voltages E sin wt and E cos wt on the deection system and electron beam of the cathode ray tube reconstruct or synthesize the rotating vector E from its orthogonal components and cause the spot to execute the small search circle Q, the location of the center of which is determined by voltages from potentiometer's 20a and 2Gb.

The image of the face of the tube 10 on which the spot of light S is moving is focused by a lens 11 on a curve display means which may comprise a stencil or other member 12 on which is impressed a curve 13 that in FIG. 1, is shown by way of example only, as being the shape of the outline of a kidney bean. Stencil 12 may be transparent or translucent and curve 13 opaque, in which case the light transmitted by the stencil is collected by a second lens 14. -I-f member 12 is opaque and reecting, curve 13 may conveniently be its only nonreecting portion, in which case lens 14 should be positioned on the same side of the stencil as lens 11 and offset therefrom in order to collect reilected light.

Of course, it should be understood that any equivalent curve display arrangement can be used. In particular, the transparent and opaque, or reflecting and non-reiecting portions of member 12 may be interchanged. Member 12, may for example, be either a positive or negative photographic film, or a portion of intermittently moved roll of microfilm. In any of these arrangements, the curve 13 is defined by the boundary between adjacent regions of member 12 which have different optical properties. Such a boundary exists, for example, along a line separating regions of different optical density, grey scale, or transparency in a photographic negative. In general such a line represents an equi-density or constant grey level line and the gradient or rate of change of density or grey level may be either continuous throughout the area including the line, or (in the special case of two-tone or black and white images) the line may corresopnd to a discontinuity in the grey scale. For purposes of clarity of discussion the latter special case of black and white or two tone definition will be assumed in the remainder of the specification. It should, however, be understood that the system may be used to read either type of material. If the system is used to follow along an equi-density line in an image having a continuous density gradient or variation of grey level, the only difference in operation is that the output of the photoelectric transducer (to be described in detail below) becomes a continuously varying waveform, such as sinusoid, rather than a series of pulses. Both types of output, however, contain essentially the same information, as will become apparent from the discussion below.

The curve display means and the search surface on which the spot of the electron beam device is focused are positioned in what may be termed reciprocally imaged relationship. By this term is meant that if the search surface, which may for example be the screen of cathode ray tube 10, is considered as an object, then it willY be imaged on the curve display means and conversely, if the curve display means is considered as an object then it will be imaged on the search surface in accordance with well known laws of optics. Of course, the limiting case of reciprocally imaged relationship occurs when the curve display means is a mask or other display medium placed immediately on or adjacent the search surface so that points on the curve display means and points on the search surface directly have the oneto-one correspondence which in other arrangements is achieved by the use of an intervening lens.

The light collected from member 12 by lens 14 is focused on a transducer such as a photocell 15. Of course, tube 10, lenses 11 and 14, curve display means 12, and photocell 15 may preferably be enclosed in any convenient housing to exclude ambient or extraneous light. Transducer or photocell 15 may, for example, be a devicethe current flow through which is determined by the amount or intensity of light incident on it. When this current is caused to flow through a resistor, a voltage output is derived. Since cathode ray tube is operated at constant beam or spot intensity, the intensity of light falling on photocell and the voltage output thereof will be constant when the spot of light is traversing the background portion of the curve display means regardless of whether the background is light transmissive or not. When the spo-t crosses curve 13, however, the intensity of light to the photocell is varied to its opposite extreme and a voltage pulse will appear in its output. If the background of curve display means 12 such that light is transmitted, that is, if it is either transparent or reflecting, and curve 13 is not, the pulse will be negative going. If curve 13 is light transmissive and the curve display means 12 is not, the pulse will be positive going. In either arrangement, the voltage pulses may be amplified by an amplifier 16.

It should also be noted that cathode ray tube 1) could, alternatively, be an image dissector, image orthicon, vidicon, or any other suitable type of camera tube which may preferably be provided with any convenient electrostatic deflection system. The function of the photoelectric Vtransducer would then, of course, be incorporated as a part of the operation of such a camera tube and the video output signal of the tube would supply the pulse input signal to amplifier 16. If magnetic deflection is used, it is necessary to derive the deflection currents from a constant current source driven by the specifically illustrated voltage deflection signals.

Furthermore, if curve 13 is deposited on curve display means 12 with a medium which is opaque to electrons (such as an ink containing a dispersion of lead), an electron beam from any convenient source may be directly focused on one side of the curve display means as a search surface in which case the photoelectric transducer can be replaced by any convenient transducer having a voltage output which is a function of the incidence of electrons on the transducer. Such an arrangement is another illustration of the limiting case of reciprocally imaged relationship previously discussed in connection with the use of a mask. In any selected arrangement, it is only necessary that the search surface on which the beam of an electron beam device is focused to a spot the position of which is controlled by suitable deflection means, be placed in one-to-one correspondence or reciprocally imaged relationship with a curve display means. This may be accomplished either by the physical identity of the two surfaces, by placing them immediately adjacent each other (as when only the glass end face of the tube intervenes), or by interposing suitable optical means between the two surfaces. The transducer is then required to have an output which depends upon the positioning of the spot in one or another of the portions of the area of the search surface which corresponds to one or another of the regions of the display means, so that a change in the output of the transducer will indicate that the spot has crossed the boundary between these regions or in other Words, has crossed the curve being displayed.

Returning now to the embodiment of the invention specifically illustrated and assuming for the moment that, as shown in FIG. l, switch arm S1 is connected to termirnal 16', the output of amplifier 16 is then applied to first and second harmonic filters 18 and 19, respectively, through potentiometers 27 and 28. Of course there will be no pulse output from photocell 15 or amplifier 16 until the search circle intersects the curve. At the initial instant when the search circle Q intersects the curve, the inputs E cos wt and E sin wt from circle generator 21 are causing the spot to move in the search circle Q having its center O located at a position pxo, po determined by the outputs from potentiometers a and Zlib which are thereafter left xed in value. The magnitude of the radius of the search circle Q will be determined by the absolute value or magnitude of the vector E and may convenientlyv be made negligibly small by comparison to the length of curve 13 by suitable equal settings of potentiometers 23 and 24. In practice a search circle having a diameter of the order of magnitude of a few millimeters has, for example, been found satisfactory. Of course suitable adjustment may if necessary be made in the magnitude and phase of the outputs of the circle generator in order that the particular deflection means used will cause the spot to rotate in a circle even if the deflection system is not perfectly linear. For the present, however, we assume as noted above that the deflection system is linear in the relation between deflection and applied voltage at least to within the degree of precision desired for the overall system. The frequency of rotation of the spot S around the circle Q will of course be determined by the frequency of master oscillator 22A of circle generator 21 which serves as a clock or phase reference for the entire system. In practice a frequency of 450 kilocycles for the master oscillator has, for example, been found satisfactory.

When, as shown in FIGURE 5 the distance along a perpendicular line or normal ON-lfrom the center O of search circle Q to a tangent, L-L+ to curve 13, is less than the radius `OG of the circle Q, the spot S will cross curve 13 twice per revolution around the circle Q as for example at points G and H. The segment GH of curve 13 shown in FIG. 5 may, for example, be an enlarged view of the segment included within the search circle Q as shown in FIG. l. There is no loss in generality in taking the segment GH to be rounded as shown since a sharp corner or intersection does not exist in the physical medium in which curve 13 is drawn when it is magnified to the scale of the drawing in FIG. 5. It will be recalled that the diameter of search circle Q will normally be of the order of magnitude of a few millimeters. Even if curve 13 does come to a sharp point, however, it is immaterial to the operation of the system, since the tangent to the curve 13 is approximated by the chord GH as seen by search circle Q and the rest of the system.

It is convenient to consider the tangent L-L-lto be moving in a positive direction when it moves counter clockwise around the curve 13 and to consider the normal to be pointing in a positive direction when it leads, that is, when it is ahead of the tangent in counter clockwise rotation by On this assumption a positive normal will always be pointing toward the inside of a closed curve. The set of axes formed by L-L} and N-N-lmay be thought of as rotating with respect to the x-y axes of the tube face as the center O of circle Q traces around curve 13. Of course, center O of the search circle is also the origin of the L, N axes. In order to avoid distortion of the search circle, the spot S is caused to rotate around the circle Q at a far more rapid rate than the center O of search circle Q moves around curve 13. Hence, for a single rotation of S around circle Q, the center O of circle Q may be considered to be stationary with respect to the x-y axes, or to the position OF (shown in FIGURE 5) in which vector E lies when time t equals zero, rather than to be moving along a line parallel to L-I- around curve 13. That is to say, a single rotation of S around Q may be regarded as taking a still snapshot of the motion of search circle Q relative to curve 13 during a very small time interval. Hence the angles 9G and 6H of the two points G and H at which the spot S intersects the curve 13 may be measured, as shown in FIG. 5, with respect to axis OF in the Search circles set of orthogonal axes. Of course, over a longer period of time the origin O of the search circles set of orthogonal axes, as shown in FIG. 5, will move relative to the origin C of the tubes set of orthogonal axes, but the two sets of axes will always remain parallel to each other so that angular measurements in the two are equivalent. The relationship between these two sets of axes is given at any instant by the position vector from the center C of the tube to the center O of the search circle. Like any other vector, this position vector may be expressed in either polar or rectangular coordinates. On the other hand, the relationship between the set of axes tangent and normal to the curve and the search circles set of orthogonal axes is such that they always have the same origin but rotate with respect to each other in a manner determined by the particular curve being followed.

As a result of the two intersections, G and H, of the spot with the curve the output of photocell amplifier 16 is a series of pulses as shown diagrammatically in FIG. 6 which is a waveform plot of amplifier output voltage against time. In FIGS. and 6 time is counted so that t equals 0 at the instant when the spot is at point F, or in other words, when vector E is directed horizontally along or parallel to the axis. It-vill be recalled that if 1:0, the angle wt=0, and that the cosine of zero equals 1 and the sine of zero equals zero. Therefore, at the time t equals 0, or, more generally at the time t=nT, where T is the time for one rotation and n is any integer, the vector E generated by an x component, E cos wt. and a y con; ponent, E sin wt, will have the position OF shown in FIG. 5. That-is to say, the output of circle generator 21 is used to establish a phase reference or a starting point from which time is counted. lIf one desired to use digital techniques, the voltage E cos wt could be used to control a pulse generator and cause it to emit a reference pulse when S reaches point F where E cos wt is a maximum. The pulse output of the photocell may then be considered to represent information in a pulse position modulated code, modulo 360, on an incremental time basis determined by the period T of the master oscillator. As will be seen below, however, it is not necesary to actually emit a timing pulse for each cycle in the analog computer of the present invention, since the pulses are passed through filters, the outputs of which then contain the information in the relationships of the phase of their outputs to the phase of the master oscillator. As time t increases, the angle wt increases. When wt equals the angle 9G, the spot S will be at point G, and a pulse G will appear in the output of amplifier 16. Similarly, when wt equals 9H, spot S will he at point H and a pulse H will appear in the output of amplifier 16.

It will be recalled that w equals 21rf, where is the frequency of master oscillator 22a. A=lso f equals 1/ T, where T is the period of the oscillator 22a, so that wt equals 21rt/T. Consequently, when time t equals T, the period of the oscillator, wt equals 21r or 369, and the rotating vector is back to position OF. This is marked as point F at a time T in FIG. 6. During the next cycle from T to 2T, a similar pair of pulses G and H' will appear. The time interval from G to G is equal to T, and the time interval from H to AH' is also equal to T, which corresponds to an angle of 2r radians. The time interval from F to G is equal to 61g/ w.

If the curve 13 is not a narrow line, but rather the edge of a filled in or wholly opaque shape or area on display means 12 so that, for example, all of the area below line 13 in FIG. 5 is opaque, then the pulses G and H will merge to become leading and trailing edges of a single pulse as shown by the dotted line `in FIG. 6. When such filled in material is to be read, the switch S1 is thrown to terminal 17 so that the output of amplifier 116 is applied to a diiferentiator 177 before being applied to the filters. Differentiator 17 may simply be a series-connected resistor and condenser with output taken across the resistor. As is well known such a circuit has output whenever its input is changing and no output when its input is constant. It will consequently produce separate pulses at the leading and trailing edges G and H respectively, of the single merged pulse. Dilferentiator 17 may also include or be followed by any conventional pulse shaping circuitry to give the separated pulses a uniform shape and polarity when such lled in or solid area material is to be read.

Of course, it should again be understood that any equivalent means such, for example,- as phase compensation at a later stage of the system may, if desired, be used in place of ditferentiator 17 when filled in material is to be read.

When a closed curve is drawn as a narrow line which itself gives rise to pulses when crossed -by the spot, there are of course actually Vthree regions on the curve display means, the narrow line itself being one of these regions. The regions interior aud exterior to the line will, however, have the same optical properties. Strictly speaking, two curves are dened by such a line, one bein-g the boundary between the line and the exterior region and the other being the boundmy between the line and the interior region. The former of course corresponds to the curve which would be defined if the area within or interior to the line were filled in as a solid shape. If the diameter of the search circle is greater than the thickness or Width of the narrow line, the circle may, by techniques of the present invention, be caused to ride centered between the two boundaries of the line. On the other hand, if the thickness or width of the line being read is greater than the diameter of the search circle, the search circle may be caused to ride along either the inside or outside boundary depending upon where the curve is initially picked up. In the latter case, the line is of course treated as a filled in or solid shape.

In either position of switch S1, apropriate to the type of material being read, pulses G and H will be positioned at points G and H as shown in FIGS. 5 and 6. Furthermore, the series of pulses G, G' etc. has, as a fundamental or first harmonic, a sinusoidal voltage component of frequency f equal to 1/ T, as does the series of pulses H, H etc. Here, as noted above, T is ythe period of the master oscillator. For the purposes of this specification, both sine and cosine terms will be called sinusoids since it is well known that'they are equivalent to within a constant term. It can be shown that the sinusoidal fundamental of pulses of G, G etc. can be represented by the expression, E1 cos (WH-0G), since, as best seen in FIGS. 5 .and 6, this fundamental is of the same frequency as the horizontal deflection voltage, E cos wt, of search circle Q but is displaced in phase from it by the angle 6G. That is to say, E cos wt is a maximum at point F and the sinusoidal fundamental of the pulses G, G is a maximum at point G displaced from point F by the angle 0G or, the time interval HG/w. Similarly, the fundamental due to the pulses H, H can be represented as, E2 cos (WH-0H). The amplitudes El, E2 will be equal to each other but will not in general be equal 'to the amplitude E. As has been mentioned heretofore, the pulse output voltage from switch S1 is applied to band pass filters 1S and 19.. Filter 18 is designed to reject harmonies above the first or fundamental and to transmit only voltage components having a frequency equal to the fundamental first harmonic frequency, l/T, of these pulses. The output of filter 1S is a sinusoid consisting of the sum of voltages El cos (WH-9G) and E2 cos (wt-l-GH). From a well'known trigonemetric formula for the sum of -the cosines of two angles, and from the fact that E1 equals E2, it follows that,

This expression, therefore, represents the output voltage of lter 18. For convenience, this latter expression may be rewnitten as (5) E3 cos (wt-l-H) This is also a sinusoid having a variable amplitude E3 equal to 2E1 cos 1/2(0H-0G), having a constant angular frequency w `equal to that of master oscillator 22A, and having a variable phase angle 0 equal to 1/2(0G|-6H). Therefore, as O moves and the position of intersections G and H vary, the amplitude and the phase of the signal output of filter 18 will also vary accordingly.

FIGURES 7a, 7b and 8 are similar to FIG. 5 but have the segment GH of curve 13 replaced by the cord GH 12 positive or N-laxis it is found that the amplitude E3 increases symmetrically but with a change of polarity which indicates simply that the phase angle has shifted by 180 as the center O of the search circle crosses the curve of search circle Q. From FIG. 7a it can be seen that 5 13. 'Ihe reasons for this may be seen by comparing FIG. the phase angle 0 of the output of filter 18, which equals 7b with FIGURES 5 and 7a. Basically it depends upon 1/2 (0G-Hin), represents the direction angle of the normal the fact that the angle 0 is measured, as noted above, not to, that is of the line ON perpendicular to the chord GH, in the rotating set of axes L-L-land N-N|, but in the which also bisects angle GOH, and intersects chord GH at set of axes having the vector OF as the horizontal or point I. The directional phase angle, 0, is again meas- 10 x axis. The relationship of the curve to this latter set ured counterclockwise from the horizontal reference vecof axes is changed when the search circle crosses the tor OF, or, in other words from the zero phase reference curve, even though the relationship of the curve to the time established by master oscillator 22a. Since the rotating set of axes L-l-L-N-}-N- remains the same. search circle Q is small compared to the length of the Of course, the set of orthogonal axes of which vector OF curve i3 being traced, and since the time interval of one i5 forms a part has its Orientation fixed by the Output 0f the rotation of S around the circle is small, the chord GH master oscillator of the circle generator so that this set is a good approximation to the segment GH of curve 13, or" axes will always be parallel tothe orthogonal x-y axis and the phase angle, 0 of the output voltage of `filter 13 determined by the horizontal and vertical deflection axes may be taken to represent the instantaneous direction of tube 10. Consequently, for angular measurements angle of the normal to curve 13. It will be noted that these two sets of axes are qnivalent to each other and when the approximation is exact, normal ON to chord phase angle 6 may be considered to be measured in the GH falls along the axis N-lthe normal to curve 13, and x*} orillogonal aXeS of tubo iii* AS noied apoye, ille line OL, the continuation of the side of direction angle ib relationship between the search circles axes and the tubes of `curve 13, is parallel to the tangent or `to axis L L l axes is given at any instant by the position vector from the When center O of circle Q is moving eounteroloekwise 25 center of the tube to the center of the search circle. around and parallel to curve 13, the vector velocity V Furthermore, from the immediately preceding discussion will lio along the line 0L and will have a phase anglo it is apparent that the relationship of the search circles oequalto l,[,plns lg() axes to what may be termed the curves rotating axes Furthermore, the amplitude, E3 of the output voltage ifi-L N-l'ii is given ai any iiiStent by iiie piiese ongle of filter 18, which, as noted above equals ZEI oos 0 of the output of first harmonic iilter i8. Regardless of 1/2 (0H-0G) can he expressed as best ,soon in FIG 8 in which side of the curve the circle is on, this phase angle terms of the ratio of the distance d, or line OJ, from Will always be the Phase eiiie of the exis Nl-N* rele' the Cantor 0 of the Circle Q to the Choi-d GH and the ra tive to the axis OF of the search circles set of axes. in dios r of the oiiolo Q This follows from ,the fact that other words, the 180 phase shift on crossing the curve rcthe angle between radius OG and the noi-mal ON is sulting in the change in polarity of amplitude E3 merely 1/2(0H 9G), The cosine of this angle by standard de indicates whether the center O lies along the positive or nitions is d/ r, where d is the line OJ yand r is the radius the negative branch 0f the axis N'iN Therefore: the 0G, or OH Therefore, tho amplitude E3 of the output output Voltage oi lter 18 may be considered to reprevoltago of filter 1g can he expressed, sent a vector which is always perpendicular to curve 113 40 and is always pointing toward curve i3. (6) E3=2E1 (d/r) This output signal of iilter 18 is used as the basis for It can be seen that this amplitude is a maximum when What in FIGURE l iS labeled a look-on Channel Comd=r, that is, when points G and H merge to a single prising potentiometers 27 and 32, filter 18, variable phase point lying on both curve 13 and circle Q. Of course, Shift 0i' aligning network 39, and aligning SWiiCh S2- if E3, is Zero if d is greater than r, since in this case the this Signal iS Considered i0 represent a Component of circle Q does not intersect curve 13 and pulses are not Velocity io be added in adder 35 i0 a Volidge representing produced. However, the amplitude E3 is also Zei-o when a velocity to be imparted to the center of the search circle d equals zero, that is, when points G and H lie on a diann in a direction tangential or parallel to curve 13, it is obeter of the circle Q, and consequently when the center Violis that it Will aci as a damping or Siabilizing iorrn O of the Search circle lies directly on cin-Vo 13. since its amplitude is zero when the circle is perfectly The output voltage E3 eos (wt l 9) of host haimoniC centered and since it is always pointing toward the curve iilter 18, therefore, has a Variable amplitude, E3, which Wiieii ille Circle is not perfeoily Centeredis a maximum when the search circle Q is tangent to the iii oi'dei' to obtain a Voltage representing ille aloVe curve 13 and which goes to zero when the search circle Q noted Velocity Parallel to die CurVo, ille oltput pulsos is centered on the curve 13. Also the phase angle of this GG" HH of Phoioeell ampiiiier i6 are applied through output is always in quadrature with the direction angle a potentiometer 28 to a second harmonic filter 19 which of curve 13. The amplitude E3 of the output of hist is a band-pass filter having electrical transmission charharmonic lter 1s is Shown in FIG, lla plotted as a acteristics such that it passes only voltage components function of the distance d between the center of the having a frequency equai to tWioo ille fundamental or search circle and the oui-vo 13, when the Center 0 of 60 lirst harmonic frequency of master oscillator 22a. It can the search circle is outside of curve 13 as shown in FIG- be rigorously shown, and ii has been empirically demon' URE 5, the distance d is considered to be negative since siraied: that ille enVeloPe of ille ouiput of ille Second ilarit lies along the negative portion of the axis N N monic filter has an amplitude, shown plotted as a function Thus the left hand half of the plot of FIG. 11a represents of distance d iii FIG lib, Which reaches a broad maxithe amplitude E3 from the point at which .this Cholo is 65 mum when the search circle is directly centered on the first tangent to the curvo down to the point where the curve. The equation of the curve plotted in FIG. 1lb can circle is centered on the curve. There is an initial abrupt be shown io be, rise in the amplitude E3 when the circle rst contacts the curve. This rise has a slight slope due to the finite (7) Ei-k Cos (2 arc cos d/r) overlapping widths of the search circle and the curve. As where E4 is the amplitude of the second harmonic, k is a we go along the negative axis toward the center where constant of proportionality which may be determined by 4the circle is centered on the curve, the amplitude E3 dethe setting of potentiometer 28, and d and r have the creases very nearly linearly and reaches zero when the meanings previously given. circle is centered on the curve. Furthermore, when the search circle is centered on the If the search circle continues across the cuive along the curve the instantaneous amplitude of the second harmonic will peak at the points G and H Vwhich then lie on a diameter of the search circle and on the curve 13. By this latter statement is meant that, except for an ambiguity of 180 which arises from the fact that we are considering a second harmonic, the phase angle of the second harmonic when the Search circle is centered is equal to the direction angle gb of curve 13 where both angles are measured with respect to the vector `OF of search circle Q. Theisecond harmonic may therefore be used to phase synchronize an oscillator 29 which has a basic frequency equal to the fundamental or lirst harmonic of master oscillator 22a. Since the amplitude of the second harmonic reaches a `maximum and has a phase angle equal to the direction angle of the curve when the Search circle is centered on the curve, the output of oscillator `29 may be used as a voltage representing the tangential velocity to be imparted to the center of the search circle to` cause it to ride along the curve centered thereon. As will be shown in detail below, 'it is possible by adding first harmonic signal from filter 1,8 to the second harmonic signal from filter 19 to` eliminate the 180 ambiguity in the phase synchronization of oscillator 2,9 and thereby determine Whether this velocity voltage vector is pointing clockwise or counterclockwise around the curve.

Filter 19 and oscillator 29 are shown in FIG. l as forming part of what is termed a"progression channel. It will be recalled that the output of the lock-on channel including iirst harmonic filter 18 is a voltage which goes to `zero amplitude when the circle is perfectly centered, Whichvincreases proportionally to the distance of the center of the circle away from the curve, and the phase angle of which is such that it is always in quadrature pvith and ypointing toward the curve. Therefore, when afterV suitable alignment byy phase shifting elements 30 and 31 to be described in detail below, the outputs of the progression channel and the lock-on channel are combined in an adder 35, the output of the adder is a voltage which may be used as an analog polar representation ofa velocity tobe ,imparted to the center of the search circle to vcause it to stably followaround curve 13 at linear speeds which have, for example, been measured at as high as twentyive Vfeet per second. T he lock-on channel, the progression channel, land the adder are shown in FIGURE l as enclosed within a dashed line block 36 and, taken together, may be considered to constitute a velocity voltage generator 36 having an output voltage `V cos (writ-ip) as shown in FIG. 14. Here it is to be understood that the phase angle pwill be .equaleither to direction angle tp of curve 13 or to b4-.180 in accordance with switch selectable phase shift of ihr/2 introducedV by element 31E-in generator 36 whereby thedirection, clockwise or counterclockwise, of motion around the curve may be determined. The precise mannerhin .which this is accomplished will be ldescribed in detail below after a brief summary of the overall operation of the system of FIG. 1.

Theoutput voltage Y cos (wt-{A-q) may be applied to anamplitude ycontrtnlling circuit such as a clipper or autoinatic gain controlled` amplifier 49 since in inany applications of the curve follower itis desirable to trace Varound the curve at constant linear speed. It should, however, be

' understood `that 'amplifier 49 may equivalently be replaced by anydevice which controls the amplitude V without affecting the phase angle gp. Furthermore, the amplitude V may be constrained to haveeither a fixed value as shown or to vary in any desired manner. The voutput of amplifier 49 is shown applied to a block 50y which will be described in detail below. Block 50 may be termed a resolver and integrator since it resolves the A.C. velocity voltage into orthogonal D.C. components which are then integrated by integrators Se and 55. The integrator outputs are incremental position voltages which are applied through the deiiection amplifiers to the cathode ray tube to actually formation in the outputs of first and second harmonic 14 filters 18 and 19 thereby closing theA damped positionservo loop and causing the Search circle to travel around curve 13.

The incremental position voltage Apx and Apy may also conveniently be applied to adders 60 and 61 respectively to which the initial position voltages 17o and py0 are also applied. The outputs of these adders will then be the instantaneous D.C. or orthogonal components of the position vector from the center of the cathode ray tube to the center of the search circle. When the center of the Search circle is constrained to move at constant speed, the outputs of adders 60 and 61 will also represent a transformation of the function y=f(x) plotted as curve `13 on stencil 12 to a parametric representation, x=x(s) and y=y(s) of the same curve 13, Where the parameter s, of course, is the arc length along curve A1?). If these outputs are applied to a second or monitor cathode ray tube 62, the curve 13 itself will be reproduced on the screen of tube 62. The Search circle voltages from circle generator Z1 are, of course, applied only to tube 10 and not to tube 62. It will be understood that tube `62 is merely one convenient output device and that any other desired output device may be used in place thereof. In particular the outputs of adders 6i) and 61 may be read into any convenient storage medium by analog to digital converters and the stored information may thereafter be used for example as input data to a computer or to program or control any desired automatic machinery, etc. g

Returning now to a detailed consideration of the second harmonic signal and the manner in which it is processed by the system of FIG. l, reference is first made to FIGS. 9a, 9b and 9c. in FIG. 9a search circle Q is shown outside of and tangent to curve 13 at point G and a voltage amplitude vs. time plo't of the resulting second harmonic is given. This situation corresponds to that which exists at the point marked 9a on the distance axis of FIG. 1lb. It should be kept in mind that the plot of FIG. 11brefers to the variation in the amplitude of the envelope of the second harmonic whereas a plot of the instantaneous values of the second harmonic at point 9a is shown aswaveform G in FIG. 9a. It is apparent that since circle Q intersects curve 13 only at the single point G, there will be only one pulse G per revolution of the spot around the circle and that, as shown, the output of second harmonic filter 19 has two positive peaks for each revolution of spot S around circle Q, alternate peaks occurring at a time corresponding to point G.

In FIG. 9b a similar type of diagram is shown yfor the situation where the 4Search circle has moved toward curve 13 so that center 0 is now located at point 9b on the distance axis of FIG. 1lb. It is now apparent that there are two intersections of the search circle with the curve, specifically at points G and H, for each revolution of the spot around the circle. The output of ilter 19 will therefore be the sum G-i-H (shown 'as a solid line waveform) of the two individual wave trains shown as the dashed line waveforms G and H in FIG. 9b. Of course, waveform G originates or peaks at a time corresponding to point G and similarly waveform H originates or peaks at a time corresponding to point VH. The phase relationships of the individual waves G and H will thus be determined by the relative positions on 'search circle Q of the intersection points G and H and the resulting phase relationships will in turn determine the amplitude of the envelope when the two individual wave trains add. It is this resulting amplitude which is plotted in FIG. 1lb. When center O is positioned so that d equals r/ \/2 or `approximately 0.7r (as labeled in FIG. 1lb), the angle GCH shown in FIG. 9b will be exactly 90 and the two individual wave trains G and H will be exactly ,180 out of phase on the second Vharmonic phase scale s o that they will destructively interfere or cancel each other giving a zero amplitude for the envelope as shown in FIG. 1lb. The situation actually shown in FIG. 9b is for a value of d slightly less than 0.71', as shown by point 9b in FIG. `l'lb. Due tothe change in 15 phase relationships which occurs when center O passes through the point for which d equals 0.71', the polarity of the amplitude of the envelope at a point such as 9b will be positive whereas for points such as 9a it will be negative.

The situation which exists when the center O of search circle Q lies on curve 13 or is at point 9c on the graph in FIG. 1lb is illustrated in FIG; 9c. Here it is apparent that the angle GOH on the search circle has become equal to 180 which is equivalent to 360 on the phase scale of the second harmonic, so that the individual second harmonic component waves G and H are in phase with each other and will reinforce each other when added, thereby making the amplitude of the envelope a maximum as shown in FIG. 1lb. As may be seen in FIG. 9c, however, two separate instantaneous maxima will occur at points 63 and 64 corresponding, for example, in time to the points G and H', that is, to each of the two intersections of the spot S with the curve 13 during one revolution of the spot around the circle Q. Consequently, if one attempts to use solely the second harmonic output of filter 19 to phase synchronize first harmonic oscilaltor 29, there will be two phase stable modes of possible oscillation depending upon which of the two maxima the oscillator happens to lock onto initially. l One circuit which may, for example, be used for oscillator 29 is shown in FIG. 12, and comprises a Colpitts type oscillator circuit which may, for example, use a 616 double triode tube 64. The second harmonic output of filter 19, as well as a portion of the first harmonic output of filter 18 derived through variable phase shifting element 30, potentiometer 33 and phase shifting element 34, are applied through a coupling capacitor 65 and across a grid bias resistor 66 to the grid of the first half, 64a, of tube 64. It will be recalled that the first harmonic output of filter 18 has a phase angle which is in quadrature with the direction angle of curve 13. Variable phase shifting element or network 30 is used only for alignment purposes to correct for slight inaccuracies in the system and may be ignored for the purposes of the present portion of the discussion. The quadrature first harmonic output of filter 18 is then applied through potentiometer 33 and any convenient phase shifting network 34 which selectively introduces a phase shift of plus or minus 90, that is, plus or minus vr/2 radians. The phase of the rst harmonic signal will now be represented by a vector which is parallel or antiparallel to the side of the direction angle of curve 13 depending upon whether the phase shift is plus or minus. Unlike the second harmonic, the first harmonic signal reaches a positive maximum peak only once during one rotation of spot S around circle Q. Assuming that the search circle approaches the curve from the outside as illustrated in FIG. 7a, this will occur either near point H or G depending upon whether the phase shift introduced by element 34 is positive or negative. Of course, the relationships are simply reversed if the search circle approaches the curve from the inside as shown in FIG. 7b. In either event a negative or lagging phase shift of vr/Z will cause the center O of circle Q to tend to make a right hand turn from an approach along the N axis to tangential travel along the L axis.

The first harmonic output of element 34 may thusr be used while the search circle is initially approaching the curve to determine which of the two possible phase stable modes of oscillation the oscillator 29 will synchronize to or lock on to. Through suitable adjustment of potentiometers 27, 28, and 33 the maximum amplitude reached by the first harmonic at the input to the oscillator, which will occur at a point such as 67 in the graph of FIG. 11a, may, for example, conveniently be made about twice the maximum amplitude of the second harmonic which maxima will occur at points 68a and 68]; in the graph of FIG. 11b. When this is done there will be a point such as 70 along the common distance scale of FIGS. 11a and 11b at which the amplitudes of the envelopes of the rst and :second harmonics .are equal. If the center of the Search circle is outside of this point the amplitude of the rst harmonic will be large by comparison to that of the second harmonic and will predominate in their sum which is used as the synchronizing signal for oscillator 29. As the search circle begins to center on the curve, however, the irst harmonic amplitude decreases while the second harmonic amplitude increases, the first harmonic finally going to zero when the circle is exactly centered. But by the time point 70 is reached oscillator 29 is running in the desired one of its two phase-stable modes of oscillation and due to the inertia of its tank circuit ignores the alternate peak lof the second harmonic. Essentially, the second harmonic is counted down to the frequency of the first harmonic while still retaining the desired phase relation.

This action may be more clearly visualized by a further consideration of the circuit shown in FIG. 12. Sections or halves 64a and 64b of tube 64 have anodes 72 and 73 which are connected in parallel to a source 74 of B+ power and also have a common cathode 69 which is connected through an inductor 70 and resistor 71 to ground. The tank circuit of the oscillator comprises capacitors 75 and 77 and a variable inductor 75 which may be adjusted to determine the frequency of oscillation. These components should be chosen so as to obtain as high an electrical Q for the tank circuit as is conveniently possible. Common cathode 69 is connected to the junction of capacitors 76 and 77 which have their other sides connected to opposite ends of inductor 75. The tank circuit is then connected between the grid of tube section 64b and ground, and output is coupled from the tank circuit through a capacitor 78. Since the tank circuit is tuned to resonate at the fundamental first harmonic frequency of master oscillator 22a, this output will have a frequency equal to that of the first harmonic signal derived from filter 18. Furthermore, the phase of the output of oscillator 29 with respect to the phase of the output of master oscillator 22a is synchronized in the manner described above.

It is apparent that when the circle rst approaches the curve the synchronizing signal `on capacitor 65 will have a phase determined by the predominating first harmonic signal from filter 18 which is the signal that initiates the oscillation in the tank circuit. When the circle centers on the curve, the rst harmonic dies out and the second harmonic takes over the synchronizing function, However, the inertia of the tank circuit by this time will have established a preference for the correct one of the two peaks of the second harmonic occurring in each cycle of the first harmonic. The tank circuit will respond to slight or continuous phase changes of the second harmonic but will not change instantaneously through into the alternate phase-stable mode of oscillation.

The initial alignment procedure used before the system is placed into operation is diagrammatically illustrated in FIGS. 10a and 10b. This procedure is used merely to compensate for minor inaccuracies or variations in the components selected for any particular system and is simply one convenient means of insuring that the phase relationships in the system are in fact such as to produce the desired results. In order to align the lock-on channel, switch S3 in the progression channel is opened or thrown to its grounded terminal so that no second harmonic signal enters adder 35. A set of orthogonal axes comprising perpendicular, horizontal and vertical lines is actually drawn upon a transparent mask which is then positioned over the face of tube 10 as shown in FIGS. 10a and 10b so that the intersection of the lines is at the center C of the face of the tube. Potentiometers 20a and 20h are then adjusted so that the center Q of the Search circle is positioned at a point O a short distance away from either the horizontal or vertical line. When the system is properly adjusted the first harmonic signal from the lock-on channel should cause the circle to move from point O to a point O on the line and the circle should remain centered thereon and stationary. Variable phase shift element or 17' networkt); may-be"A adjusted until this desired action occursregardless of which side; of:l either the horizontal or vertical line the circle starts from as illustrated in FIG. h11.

Switch. S3' is now closed and switch S2 is opened or` thrown to its grounded terminal so that only signal from the progression channel reaches adder 35. If the circle is now positioned at a point O adjacent one of the lines asin. the, procedure for aligning the lock-on channel, it should movein a direction (indicated byk vectors V in FIG. 10b) parallel to the line and having a positive or negative direction of motion determined by the setting of the selectable 90 phase shifting element 34. This action should result at any distance d, equal to or less than radius r, away fromthe line'or with the center O initially positioned on the line. It will be noted that without the lock-011 channel in the system the circle will simply moveV parallel tothe line but willnot center upon it unless it is. initially so centered. Variable phase shifting element 31 is now adjusted until this.` desired. motion parallel to the line is obtained in either direction along either line.

lf the particularintegrators used atf54 and S5 are such that they are subject to drift. or accumulation of stray charge while the system is in a static or'non-operative condition, thissame alignment. procedure may more conveniently be carried outfby measuring the output voltages of phase, detectors 52. and 53 to ensure that they are such as. would result in the above noted action whenthe system is inA an, operative ordynamic state. For example, in adjusting the progression channel with the lock-on channel shorted outin order-to produce motion along the x axis asf shown inFIG. 10b, itwould be necessary to haver zero output fromphase detector 53 with positive polarity outputfiom. phase detector 52.V Variable phase shift element 31; may, as noted. above, beadjusted until this condition is. achieved. If the. alignment is correct exceptfor a polarity reversal. the desired polarityvmay readily be` achieved. by reversinga transformer connection or. otherwise introducing a *phase inversion at any convenient point in the. system as will be. apparent to those skilled in the art.

`Of course, during operation of the system switches S2 and S3 are'. both left closed and potentiometer 32 may now be adjusted empirically to determine the amountof lock-on signal. necessary to be. added to the progression signalto produce optimum stability or critical damping` of the system. Potentiometers 27 and 28 it will be recalled, will have been adjusted so that in conjunction-with potentiometer 33 they establish the desired ratio of irst tov second harmonic in the synchronizing signal applied to oscillator 29. With the setting of potentiometer 27 thus determined,.. potentiometer 32 is then adjusted to obtain theabove noted optimum stability of the overall system. Finally,.the linear speed at which the center of the search circle will travel along a curve is determined by the. setting of amplitude.. controlling device 49.

Automatic gain control. device 49 should preferably be a clipper type of circuitA rather than the type which actually or literally varies the gain of anampliiier by a feedback signal. since it has been found thatthe clipper type of circuit will not aifect. the phase modulation of. the signal whereas. a-, slight error. may be introduced by a feedback controlledv variation. in. gain. Alternatively, element 49 may be a balanced modulator of the type described in detail in the above noted copending application S.N.` 618,504. This` type of. balanced` modulator is a modification of the so-ca1led.Diamod circuit'described inrFIG. 1.1.8. at. page398 of volume 19, Waveforms of the Massachusetts Institute of Technology Radiation Lab-- oratory Series, McGraw-Hill, New York, 1949. The

Diamociy should, however,. be modified by tuning its` input. transformer tothe fundamental rst harmonic frequency of. master oscillator 22a.v Such. a balanced modof which are-equal to the frequency and phase of 'itsA-.C. input and the amplitude of which is` equal to` its D.C. input. When such a circuit is used,.best resultshave been` obtained by using a toroidal ferrite core transformer and exercising particular care in accurately locating the center tap thereof. This type of circuit permits one to determine the linear speed of motion of the center of thesearch: circle in response to a D.C. voltage. It should alsoV be noted that if the D.-C. input goes negative, a phase shift is produced in the output thus affording an additional control over the direction of motionclockwise or counterclockwise, of the center of the search circle. It will be recalled that a positive 1r/ 2 phase shift by element 34 will result in clockwise or counterclockwise rotation. depending upon which side of curve 13 the search circle initially approaches from. Since a change in polarity of the D.C. input to a balanced modulator type of amplitude control t9 reverses the direction of motion, it permits one to achieve either clockwise or counterclockwise: motionl regardless of which initial approachk direction the: system is using. Of course, when the circle isV constrainedV to move at constant linear speed the A.C. outputV of circuit 49 may, if desired, be rectified and integrated by an. operational amplifier or other integrating device not shown. The reading of the integrator will then be a measure of the total arc length traveled over by the cen-V ter of the search circle during the time of integration. This circuitry is not shown since it comprises only one of many possible applications of the system and is not necessary to the basic operation of the system.

The output of amplitude controlling circuit 49, whichv will be the signal V cos (wt-i-qb) with its amplitude V` now determined independently, is applied to Va pair ofi phase detectors 52 and 53. Each of these phase detectors.' may, for example, comprise a circuit which is fullydis-A closed and claimed in the above noted copending application S.N. 618,504. Considered as a black-box the; phase detector may be said to be a circuit having two A.-C. input voltages of the same frequency, one beingl called a signal voltage and the other being called a;v carrier voltage, and further having a D.C. output voltage. which is equal to the amplitude of the signal voltage times a sinusoidal function of the angle of phase difference between the two A.C. input voltages. As used in the present system the velocity voltage output signal of amplitudeY controlling circuit 49. is the input signal voltage to each of phase detectors 52 and 53. Carrier input voltages for phase detectors 52 and 53 are the voltages E cos wtJ and E. sin wt derived from circle generator 21. In the operation of the'circuit the instantaneous amplitude of the signal input voltage is sampled once each cycle at a timei determined by the maximum value of the carrierv voltage. which in this case is derived from the master oscillator orV circle generator. Where the input signal to the phasey detector is a cosine wave andthe carrier signal on the back-to-back sampling diodes of the circuit as disclosed in the above noted copending application is also a cosine wave, the D.C. output is equal in magnitude, and throughout all quadrants has the correct polarity of the cosine of; the angle of phase difference between the two A.C. inputs;` Similarly where the input signal is a cosine waveA and the: carrier is a sine wave, the D.C` output is proportional to the sine of the angle of phase difference throughoutall'. four quadrants. Any circuit having these propertiesV may be used for phase detectors 52 and 53. As noted above, however, one speciic example of such. a circuit is disclosed and claimed in the copending Brouillette-lohnson application. The specic details of this circuity do not form apart of the present invention.

From the above stated properties of the phase detec tor circuits 52 and 53 it is apparent that their D.C. outputs will, in. accordance with equations 1A and 1B above,

represent the x. and y components of the velocity vectorf Y where. this vector is resolved in theV set` of axes determined by the search circle. These axes, it wilk be"- recalled, always remain parallel-to the x and y orthogonal axes of tube 10. These output voltages are therefore labeled in FIG. l as vx and vy, and, from a consideration of Equation 2 it is apparent that these voltages are also equal to dx/ ds and dy/ds when the center of the search circle is moving at constant speed. Here, of course, x and y are the coordinates of a point traversing curve 13 at constant speed and the derivatives indicated are the rates of change of these coordinates with respect to arc length s along curve 13. These voltages are inherently functionsof time, but since it is well known that speed equals arc length or distance divided by time it follows that when the center of the search circle moves at constant speed these voltages become proportional functions of arc length, and, like the voltages from adders 60 and 61 may if desired be read out of the system or taken as an output for any desired purpose.

These voltages may, for example, be desired in systems such as that disclosed and claimed in the copending application S.N. 618,504 of 4Charles W. Johnson and Paul Weiss, entitled, Form Recognition System, filed concurrently herewith and assigned to the same assignee as the present application. If the electronic curve follower of the present invention is used in place of the electromechanical curve follower described therein, the computations disclosed therein may be made upon voltages read out of the present system in order to determine various invariant properties of a curve which may be compared with stored standard values in order to recognize an unknown form. This again however, is merely one possible application of the system of FIG. l. It should, however, be noted that in this type of application the linearity of the deflection system of tube is not critical. The system will operate as a follower since linearity errors are compensated for bythe servo loop.- Furthermore, 'the stored standard values maybe computed from the same system used to'read an unknown form. It is only the quantitative accuracy of data read from curve 13 for use with independent external apparatus which will be affected by the degree of linearity of the deflection system of tube 10.

In the system of FIG. l the voltages vX and vy are respectively applied to integrators S4 and 55. These integrators are preferably operational amplifiers of the type commonly used in analog computers. In general they comprise high gain D.C. amplifiers having a resistive input impedance element and a capacitive feedback im.- pedance element.

The outputs of integrators 54 and 5S are incremental position voltages Apx and Apy, respectively, which are applied to the x and y deflection amplifiers 25 and 26 and which cause the center of the search circle to move as these voltages vary. These incremental position voltages may also be applied to adders 60 and 61 in order to reproduce curve 13 on the monitor scope 62 when they are added to the initial position voltages pxu and pyo from potentiometers 20a and 20b. Of course, these initial position voltages need not be derived from separate potentometers, but could be the initial conditions read into the system by suitably charging the capacitors of integrators 54 and 55 in the `manner common in the analog computing arts. In this case, the outputs of integrators 54 and 5S and the outputs from the circle generator are all that one needs to apply to the deflection amplifiers, and the outputs of they integrators themselves, when applied to the'deect'ion system of monitor tube 62, would cause curve 13 to be reproduced thereon. Which of these procedures is most convenient depends solely upon the particular application fo'r which one intends to use thecurve follower. A

Many of the phase relationships existing in the system as well as the above discussed relationships between the various sets of coordinate axes can be conveniently visualized by reference to FIG. 13 which is a diagrammatic drawing (with Search circle Q drawn to a greatly enlarged scale) of the special case in which the curve 13' to be followed is itself a larger circle having its center at the center C of the set of x and y axes determined by the deection system of tube 10. Variations from this special case of a circular curve produce different quantitative values of the variables involved, but all depend upon the same basic principles of operation illustrated therein. v

In FIGURE 13 it is assumed that the center of the search circle is initially positioned at a point O1 lying at the intersection of curve 13 with the x axis. At this point the search circle is labeled Q1 and a position vector P1 extends from the center C of tube 10 to point O1. S-ince this position vector lies along the x axis, it is simply the voltage pxo, the voltage py0 being equal to zero in the special case. The prime and subscripts are used in the notation for vector Pl to distinguish it from the vector P which in FIGURE-2, is shown as being the vector from the center C of the tube to spot S. In FIG. 13, the vector P would consist of the sum of the vectors El and E, the Etter of which, at time t equal to zero, extends from point O1 to point F on search circle Q1. It will be recalled that the vector E is generated by the circle generator 21 which may for-example, be running at 450 kilocycles per second. The vector B1 has not herein been shown as being explicity generated electrically in A.C. or polar form, but rather exists in D.C. component form as the outputs of adders 60 and 61. These outputs, which in FIG. 13 are illustrated for the second position Q2 of the search circle, could however readily be converted to equivalent A.C. form by use of the technique of applying them as inputs to a pair of balanced modulators the outputs of which are combined in an adder. The balanced modulators would have carrier voltage inputs derived from circle generator 21 in accordance with the technique disclosed and claimed in the above noted copending Brouillette-Iohnson application.

` As spot S, which is initially at point F on search circle Q1, rotates about the search circle it intersects curve 13 at points G and H which are labeled in conformity with the notation used earlier in the specication. These intersections produce the pulses in the photocell amplifier which are applied to rst and second harmonic filters 18 and 19 the outputs of which are used to phase synchronize oscillator 29 which in turn generates a voltage representing a velocity to be imparted to the center of the search circle. It will be recalled that the second harmonic has positive peaks at both points G and H but that rst harmonic signal is initially used to synchronize rst harmonic oscillator 29 to a phase stable mode of oscillation determined by point G for counterclockwise rotation. When search circle Q is small, as it is in practice, diameter GH of the search circle is a very good approximation to a segment of the circle or to curve 13 to be followed, and velocity vector V will lie along this diameter and be tangent to curve 13 2 Although the search circle may not remain in mathematically perfectly centered relationship for curves other than straight line segments as it travels around the curve to be followed, it will be noted by reference to FIGS. 11a and 11b, that if it stays within the distance d1 on either side of the curve indicated by the dashed vertical lines, the second harmonic predominates in the synchronizing signal and a change in polarity of the rst harmonic resulting from displacements on opposite sides of the curve will not change the phase stable mode to which the oscillator 29 was initially locked. This condition is readily achieved through appropriate adjustment of potentiometer 32` for optimum system stability.

In FIG. 13 search circle Q is shown in eight different positions in order to illustrate what happens as it travels around the curve 13. Of course it will be understood that the number of times spot S rotates about the search circle for one rotation of the center of the Search circle about closed curve 13 is primarily determined by the` 21 relationship orA ratio ofthe frequency of master oscillator 22a' to the ratio of speed or magnitude oflvector V tothe total arc length s'of curve 13. Thus, if master ois-cill'ator 22a has a frequency ofv 450 kilocycles per second andV the center of the Search circle travels aroundY curve 13' one hundred times per second, spot S may be thought of as rotating. around the search circle 4,500 times in one complete revolution around curve 13. TheV eight posi-V tions shown are therefore merely representative samples at discrete intervals of'what is in fact a continuous process.

When the center of the searchcircle has reached the point O2 on curve 13', V will have a phase angle q meas-- ured, as shown in FIG. 13, with respect to the search circles axis established by the vector E when it is in the position OF. As noted earlier, the search circles axes always. remain parallel to the tubes x--y axes even though the center of the search circle moves, since the Search circles axes are in effect merely the zero time reference established by master oscillator 22a marking the completion of one rotation of spot S around the circle circle.

Similarly, position vector P'1, as dei-med above, will have moved to a position P'2 'and will have a phase angle A2 measured equivalently 'h1 either the Search circles set of axes or the orthogonal x--y axes of tube 10. If the. vector P2 were explicitly generated as an A.C. voltage as suggested above, the angle h2 would be its phase angle at point O2. Alternatively, the D.C. orthogonal cornponents of this vector may be obtained, as-shown in FIG. 1, as the outputs of adders 60 and 61 and, at point O2, will have values as shown in FIG. 13 of x2=x(s) and y2=y(s). That is to say, the values will be functions of the arc length of the particular curve being traced when the search circle moves alongk curve 13 at constant speed.

It should be noted that as the search circle Q progresses around the curve 13', the point G advancesV in phase around the search circle, so that when circle Q is back to point O1 after one complete revolution around circle 13' the point G is back where it started from on the'searchcircle; This, however, implies that G has ad vanced 360 or one complete phase cycle per revolution around the curve being traced. Thus, if the master os-V cillator frequency is 450 kilocycles per second and the center of the Search circle moves around circle 13 100 times per second, spot S will make not 4,500 complete rotations as initially suggested above, but rather 4,501 complete rotations around the Search circle for each complete revolution around curve 13'. That is to say, the output of oscillator 29 is being frequency modulated from 450 to 450.1 kilocycles per second by a phase stabllized and phase controlled system the phase control signals for which are derived from information embodied in input pulses of a pulse position modulated code the basic interval for which is established by the master oscillatorhaving a frequency equal to the center or carrier frequency of'olscillator 29. The instantaneous frequency of oscillator 29, of course, is equal to its rate of change of absolute phase. The instantaneous phase of oscillator 29 with'respect to the master oscillator, however, is the angle if as illustratedv in FIG. 13. It should also be noted that the totalvoltages, x(s-) and y(s), on the deection plates of tube 62 may also be considered to be quadrature com-V ponents of a voltage P cos (wt-Ht) the carrier in this case beingderived` directly from the master oscillator or circle generatorl by'the'useof balanced modulators as suggested above; In general the voltage P cos (wt-l-) would be both amplitudeand frequency modulated.

The: full meaning of the statement that the; system is phase; stabilized becomes` more apparent when curves other.- than; the perfectly centered circle` 13' are consid-- ered.` For: that; special case considered above', the anglev -,fof course, is'` always equal to the angle A plus 90. However, this isobviously not ini general .true of all curves. I f, for.' example, the* curve being followed includes a` straight' line segment, Iangle rp remains constant along the:

straight line whereas.' angle A. will not remain. constant.`

unless the straight linev segment isa radial line passing.

through the centerV C of the tube 10. Also ifcircle 13 were simply moved along'the x axis to thepointwhere it is centered on the x axis and tangent to the y axis, then the .angle A will oscillate between i45 while angle fr changes through 360. Furthermore, even if the signal` V cos (WH-4;) is entirely cut olf or goes to zero, the tangent of the angle A is still determined as a constant value by ratio of the outputs of integrators 54 and 55 which provide what may be termed a phase memory for the systems position vector. It is this action as well as the synchronization of osciliator 29 to a phase stable mode which is referred to in saying that the system may be termed a phase stabilized method of frequency modulation phase controlled in response to input signals embodied in a pulse position modulated code.

When thesystem is used as a curve follower as shown in FIG. 1, the pulse position modulation is determined through the coaction of the electron beam deviceor, cath-- ode raytube 10, curve display means 12, and photocell 15. It will, however, be obvious to those skilled in the art that other means of modulating the position. of the input pulses within a time interval set by master oscillator 22a may readily be employed and that the system. may be extended to applications which do not necessarily include the feedback of information which creates the closed servoloop of the curve follower. iFor example, when initial position conditions or voltages are set into integrators 54 andY 55, (or on potentiometers 20a andv 2Gb) thev photocell or transducer may be omitted andv oscillator 2% may be phase controlled in response to signals representing any known information. The system may Ithen be used as a curve generator or to simulate the motion` of a particle moving (fromthe established initial position) with a velocity having a direction given by the phase mgle of oscillator 29 and a magnitude as determined by amplitude control circuit 49. Of course, in such applications the outputs of integrators 54 and 5S may be applied either to a cathode ray tube or to any. other convenient output device such as a plotting table. In this type of use, of course, the search circle is not explicitly displayed and the system isV not operating as a servo loop as is the. case when. the system isiused as a curve follower;

While the principles of the invention have now beenl made clear in an illustrative embodiment, there will be' immediately obvious to those skilled in the artl many modications in structure, arrangement, proportions, the elements and components used in the practice of the invention, and otherwise, which are particularly adapted forV specific environments and operating requirements without departing from these principles. The appended claims are therefore intended to cover and embrace any such. modifications,.within the limits only of the true spirit and scope of the invention.

What I claim as newand desirel to secure by Lettersv Patent of the United States is:

l'. An electronic curve follower comprising curve4 display means, an electron beam device havingelectront beam producing'means, means to focus said.beam;to: ai `scanning spot on a search surface; and` means to deect. said beam and said spot; means placing said curve display meansand said searchsurface in reciprocally imaged. relationship; means' to apply first and second sinusoidal voltages to said deec-tion means, said first and second sinusoidal voltages each having thesame. frequency andV respectively having -amplitudes and phase angles such that said deflection means causes said spo-t to move in asearch circle the diameter ofwhich is small by comparfson to thelengthof said curve on said searchsurface; meansco-acting with said electron beam device .and said curve display means to derive at least one signal voltage con-v taining information determined byV the relationship be-` tween said curve and :said circle; an oscillator tuned to the frequency of said first and second sinusoidal voltages, means to apply said signal voltage to control the phase of the output of said oscillator, means to derive third and fourth voltages from the output of said osclllator, and means to apply said third and fourth voltages to said deflection means to move the center of said search circle along said curve.

2. An electronic curve follower comprising curve display means, an electron beam device having electron beam producing means, means to focus said beam to a scanning spot on a search surface, and means to deliect said beam and said spot; means placing said curve display means and said search surface in reciprocally imaged relationship; means to apply first and second sinusoidal voltages to said deflection means, said first and second sinusoidal voltages each having the same frequency and respectively having amplitudes and phase angles such that said defiection means causes said spot to move in a Search circle the diameter of which is small by comparison to the length of said curve on said search surface; means co-acting with said electron beam device and said curve display means to derive first and second signal voltages, said first signal voltage having a frequency equal to the frequency of said first and second sinusoidal voltages, said second signal voltage having a frequency twice the frequency of said first and second sinusoidal voltages; an oscillator tuned to the frequency of said first and second sinusoidal voltages; means to apply at least said second signal Voltage to control the phase of said oscillator; means to add said first signal voltage to the output of said oscillator to form a voltage which is an analog representation of a velocity to be imparted to the center of said search circle; means to integrate said velocity voltage; and means to apply the output of said integrating means to said deflection means to move the center of said search circle along said curve.

3. Apparatus as in claim 2 and further including means to constrain the magnitude of said velocity voltage to a predetermined constant value whereby said center of said search circle is caused to move along said curve at constant speed.

4. An electronic curve follower comprising curve display means, an electron beam device having electron beam producing means, means to focus said beam to a scanning spot on a search surface, and means to deflect said beam and said spot; means to place said curve display means and said search surface in reciprocally imaged relationship; a circle generator including a master oscillator having a first sinusoidal voltage output and means to derive from said output a second sinusoidal voltage having the same amplitude and frequency as but differing in phase by 90 from said first sinusoidal voltage; means to apply said first and second sinusoidal voltages to said defiection means to cause said scanning spot to execute a search circle the diameter of which is small by comparison to the length of said curve on said search surface; means co-acting with said electron beam device and said curve display means to derive a plurality of voltage pulses containing information determined by the relationship between said curve and said search circle; means to apply said voltage pulses to first and second filters; said first filter having electrical transmission characteristics such that it passes only voltage components having a frequency equal to the fundamental first harmonic frequency of said master oscillator, said second filter having electrical transmission characteristics such that it passes only voltage components having a frequency equal to the second harmonic of the frequency of said master oscillator; a second oscillator tuned to the fundamental frequency of said master oscillator; means to apply the output of said second harmonic filter to control the phase of said second oscillator with respect to the phase of said master oscillator; means to add the output of said first harmonic filter to the output of said second oscillator to form a voltagewhich is an analog representation of a vector velocity to be imparted to the center of said search circle; means to derive voltages representing orthogonal components of said velocity voltage, means to integrate said components of said velocity voltage; and means to apply said integrated voltages to said deflection means to move the center of said search circle along said curve.

5. An electronic curve follower comprising an electron beam device including electron beam producing means, means to focus said beam to a scanning spot on a search surface, defiection means to control the position of said beam and said spot; curve display means having at least two regions of different optical properties, the boundary between said regions defining said curve; means placing said curve display means and said search surface in reciprocally imaged relationship to each other; transducer means; said transducer means, said curve display means, and said search surface being so constructed and arranged that the magnitude of the electrical output of said transducer has one value when said spot is positioned in a first area of said search surface corresponding to a first of said regions of said curve display means and has a different Y value when said spot is positioned in a second area of said search surface corresponding to a second of said regions of said curve display means whereby a change is produced in the electrical output of said transducer means when said spot crosses the boundary between said first and second areas; a circle generator comprising, a master oscillator having a first sinusoidal voltage output, and means to derive from said output a second sinusoidal voltage having the same amplitude and frequency as but differing in phase by from said first sinusoidal voltage; means to apply said first and second sinusoidal voltages to said defiection means to cause said scanning spot to execute a search circle the diameter of which is small by comparison to the length of said curve on said Search surface; means to initially position the center of said search circle so as to cause said search circle to intersect said curve; a second oscillator tuned to the frequency of said master oscillator; means to apply the output of said transducer to first and second band pass filters, said first and second filters respectively having electrical transmission characteristics such that they pass only voltage components having frequencies equal to the first and second harmonics respectively of the frequency of said master oscillator; means to apply a voltage derived from the output of said second harmonic filter to control the phase of said second oscillator with respect to the phase of said master oscillator; means to add a voltage derived from the output of said first harmonic filter to the voltage output of said second oscillator to form a voltage which is an analog representation of a velocity to be imparted to the center of said search circle; means to resolve said velocity voltage into orthogonal components taken with respect to a set of orthogonal axes the orientation of which is determined by said master oscillator, means to integrate said orthogonal components of said velocity voltage, and means to apply the outputs of said integrating means to said deflection means to move the center of said search circle along said curve.

6. Apparatus as in claim 5 and further including means to constrain the magnitude of the output of said second oscillator to a predetermined constant value whereby said center of said search circle is caused to move at constant speed.

7. Electronic curve generating apparatus comprising a master oscillator, a second oscillator tuned to the fundamental or first harmonic frequency of said master oscillator; means to derive a control signal in the form of a series of voltage pulses, the positions of said pulses within a sequence of intervals of time each equal to the period of said master oscillator being a measure of the direction' angle of said curve to be generated; means to vary the phase of said second oscillator in accordance with Vthe variation of the position of said pulses within said time intervals; first and second phase detecting means, each of said phase detecting means havingthe output voltage of said secondoscillator appliedthereto as a signalinput voltage, said first phase detecting means' having the'foutput of saidfmaster oscillator applied' thereto as a carrier input voltage, said' second phase detecting means havinga ,voltage equal" in frequency to butdiffering by 90' in phase from the output of'said master oscillator applied'thereto as a carrier input voltage, said first and second 'phase detecting means each having a direct'current output voltage; saidoutput voltages respectively beingequal to, thegamplitude of said signal'input voltage to said' phase detecting means times a sinusoidalfunction' of the.v angle" of phase difference between said"carrier input'voltage and: said' signal input voltage to said phase detecting means;'fi'rst and second integrating means. having saidfdirect current outputs ofsaid phase detecting meansrespectivelyapplied theretojand an output device to which the output voltages of said integrating means are applied to produce a representation of said curve.

8. In an electronic analog computer, a ymaster oscillator having a first sinusoidal voltage output, means to derive a second sinusoidal voltage of the same frequency as and differing in phase by 90 from said first sinusoidal voltage; a second oscillator tuned to the fundamental first harmonic frequency of said master oscillator, the output of said second oscillator being an alternating current signal voltage the amplitude of which represents the magnitude of a first vector quantity and the phase angle of which measured with respect to the phase of the output of said master oscillator represents the direction angle of said first vector quantity in a set of orthogonal axes the orientation of which is determined by said master oscil lator; means responsive to an input signal to vary the phase of the output of said second oscillator with respect to the phase of the output of said master oscillator; first and second phase detectors, means to apply the output of said second oscillator to each of said phase detectors as a signal input voltage, means to apply said first and second sinusoidal voltages respectively to said first and second phase detectors as carrier input voltages, said first and second phase detectors respectively having first and second direct current output signal voltages representing orthogonal components of said first vector quantity, and means to operate upon said first and second direct current signal voltages to derive a represen-tation of a second vector quantity.

9. An electronic curve follower comprising curve display means, an electron beam device having electron beam producing means, means to focus said beam to a scanning spot on a search surface, -and means to deflect said beam and said spot; means to place said curve display means and said search surface in reciprocally imaged relationship; means to apply first and second sinusoidal voltages to said deflection means, said first and second sinusoidal voltages each having the same frequency and respectively having amplitudes and phase angles such that said deflection means causes the spot to move in a search circle the diameter of which is small by comparison to said curve on said search surface; means cot-acting with said electron beam device and said curve display means to derive first and second signal voltages, said first signal voltage having a frequency equal to the frequency of said first and second sinusoidal voltages, said second signal voltage having a frequency twice the frequency of said first and second sinusoidal voltages; an oscillator tuned to the frequency of said first and second sinusoidal volt.- ages: means to selectably shift the phase of a first portion of said first signal voltage by plus or minus 90, means to apply both the output of said phase shifting means and said second signal voltage to control the phase of said oscillator; means to add a second portion of said first signal voltage to the output of said oscillator to form a voltage which is an analog representation of a velocity to be imparted to the center of said search circle; means to integrate said velocity voltage; and means to apply the output of said integrating means to said defiection means 26 to cause the center ofi saidA searchycicleto' move, along said curve, said motion being'clockwiseorcounterclock= wise around said' curveY in accordance with'whether said selectable phase shifting'means introducesaVA positive' or negative phase shift. i

l0. Apparatus for producing a' frequency modulated electrical signal' comprising, a master oscillator, means co-acting with said master oscillator to derive an electrical signal voltage which hasl a fundamental or first harmonic frequency equal to the frequency ofsaid master oscillator and which is phase modulated over a range greater than 360 in accordance with inputinformation; first and secondbandxpassrfilters having electrical transmission characteristics such that they pass only voltage components having frequencies equal to the firstand second harmonic frequencies respectively of'said signal voltage; a second oscillator tuned to the fundamental or first harmonic frequency of said master oscillator; means to apply the outputs of said first harmonic and said second harmonic filters to phase synchronize said second oscillator to said phase modulated signal whereby the output of said first harmonic filter preselects one of two possible phase stable modes of oscillation determinable by the output of said second harmonic filter so that when the output of said first harmonic filter becomes small said second oscillator will continue to operate in said preselected phase stable mode to follow the changes of phase of said modulated signal, thereby causing a phase controlled modulation of the frequency of the output of said second oscillator.

11. Apparatus for servocontrolling an electron beam comprising, means for generating an electron beam, means for focussing the beam to a spot on a search surface, means for displaying a curve to be followed on the search surface, means for deflecting the beam so as to move the spot in a small search circle at a predetermined constant frequency, means for deriving a voltage pulse each time the spot crosses the curve, means for generating a carrier voltage having a frequency equal to the predetermined frequency of rotation of said spot, means for modulating the frequency of said carrier voltage by varying its phase in response to variations in the time occurrence of said voltage pulses, means for sampling said modulated carrier at time intervals equal to the predetermined period of rotation of said spot, means for integrating the sampled values to obtain corrective deflection voltages, and means for applying said deflection voltages to move the center of said search circle along said curve.

l2. Apparatus for producing a representation of a variable quantity by a frequency modulated electrical signal comprising; means for generating a carrier voltage of predetermined frequency, means for generating a series of pulses to represent a series of discrete values of the quantity in a pulse position modulated code, each discrete value of said quantity being expressed by the time-position of at least one pulse within a time interval equal to the period of said carrier voltage, and means for modulating said carrier voltage generating means by varying the phase of said carrier voltage in response to variations in the time-position of the pulses in said code.

13. An electronic curve follower comprising curve display means, an electron beam device having electron beam producing means, means to focus said beam to a scanning spot on a search surface, and means to deflect said beam and said spot means pacing said curve display means and said search surface in reciprocally imaged relationship; means to apply first and second sinusoidal voltages to said deliection means, said first and second sinusoidal voltages each having the same frequency and respectively having amplitudes and phase angles such that such defiection means causes said spot to move in a search circle the diameter of which is small by comparison to the length of said curve on said search surface; means co-acting with said electron beam device and said 27 curve display means to derive a signal voltage, said signal voltage having a frequency which is a multiple harmonic of the frequency of said first and second sinuosidal voltages; an oscillator tuned to the frequency of said first and second sinusoidal voltages; means to apply said signal voltage to control the phase of the output of said oscillator; means to obtain an output signal from the output of said oscillator; and means to apply said output signal to said deection means to move the center of said circle along said curve.

2,415,190 Rajchrnan Feb. 4, 1947 Manley et al May 26, 1942 15 Z8 Doll Feb. 7, 1950 Berry et al Feb. 28, 1950 Rathje Sept. 12, 1950 Sunstein Oct. 31, 1950 -Lester Apr. 17, 1951 Rieber June 19, 1951 Haviland Oct. 20, 1953 Gale June 22, 1954 Burgett June 29, 1954 Meyerho Dec. 11, 1956 Henry Jan. 13, 1959 Frantz June 30, 1959 Sunstein Jan. 19, 1960 FOREIGN PATENTS Great Britain Apr. 22, 1953 

